Howard, I agree with your analysis.
I went for a Plus 8 because I prefer the shape and look of the car to an Aero 8, but underneath there is very little difference (just a slightly shorter chassis and no boot) between a Plus 8 and a Aero 8 S4.
I'd have loved an F Type but the depreciation is frigntning, the running costs are a bit less than a Plus 8. We bought our Plus 8 for the ful list price, less the VAT, as MMC sold it as a used car, 8 months old and 15,500 mies. It now has 21100 miles on the clock.
I doubt the market value will change much over the next 5 to 10 years. Even if it does depreciate a bit in 10 years it will be worth a lot more than a 10 year old F Type.
I bought a lottery ticket today after seeing a new Mercedes AMG GT in Switzerland and reading the road test in Autocar this week. The boss has aproved the purchase, should we win £150K or more.